Fibered metal powders

ABSTRACT

Refractory metal powder compacts are sintered and impregnated with a softer metal. The compacts are reduced to rod, wire or sheet. In the process fine fibers of the metal powder are formed.

United States Patent m1 Douglass 51 May 1, 1973 [5 F IBERED METALPOWDERS [56] References Cited 75 Inventor: 1 Richard w. Douglass,Needham, UNITED STATES PATENTS Mass. 3,337,337 8/1967 Weeton ..7s/2063,029,496 4/1962 Levi ..7s/o1c. 1 [73] .Asslgneg' Wmcester, Mass-3,310,387 3/1967 Sumpt et a1 ..75/D1G. 1 22 3,291,577 12/1966 Davies6131. 05/010. 1 I 1 ed Sept 1970 2,179,960 11/1939 Schwarzkopf ..75/D1G.1 [21] App]. No.: 74,962

OTHER PUBLICATIONS Related US. Application Data Continuation-impart ofSer. No. 807,129, March 13, 1969, Continuation of Ser. No. 626,773,March 29, 1967, abandoned.

US. Cl. ..29/l82.l, 29/182, 75/200,

75/201, 75/214, 75/DIG. 1 Int. Cl. ..B22f 7/00 Field of Search ..29/182,182.1;

75/DIG. l, 200, 214, 201

Lund et al., lntemational Journal of Powder Met Vol. 2, No. 3, 1966.

Primary Examiner-Carl D. Quarforth Assistant Examiner-R. E. SchaferAttorney-Oliver W. Hayes and Jerry Cohen 57 ABSTRACT Refractory metalpowder compacts are sintered and impregnated with a softer metal. Thecompacts are reduced to rod, wire or sheet. In the process fine fibersof the metal powder are formed.

12 Claims, 10 Drawing Figures POWDERS OF F1RST METAL L CONSOLIDATE FIRSTMETAL VACUUM IMPREGNATE' WITH SECOND METAL MOLTEN SECOND METAL PatentedMay 1, 1973 4 Sheets-Sheet l POWDERS OF FIRST METAL VACUUM IMPREGNATEWITH SECOND METAL MOLTEN SECOND METAL I 215;; fgg} ELONGATE TO ROD A f IDRAW TO C RoLL To B wIRE SHEET I T it ,L

DIFFUSION D REACTION (2) AS I LEACH OUT COMPOSITE SECOND METAL l USE ASUSE AS c METAL FELT COMPOSITE A I RE IMPREGNATE r (b) SEPARATE DIFFUSIONFIBERS REACTION FlG.l

Patited May 1, 1973 3,129,194

4 Sheets-Sheet 2 Patented May 1 1973 4 Sheets-Sheet s FIG FIG

FIG

FIG, 6

Patented May 1, 1973 4 Sheets-Sheet 4.

FIG

FIBERED METAL POWDERS This application is a continuation-in-part ofapplication Ser. No. 807,129, filed Mar. 13, 1969, which is acontinuation of application Ser. No. 626,773, filed Mar. 29, 1967, nowabandoned.

Other related copending applications are Ser. No. 59,555 filed July 30,1970, Ser. No. 869,404 filed Mar. 13, 1969, as a division of 626,773,Ser. No. 839024 filed July 3, 1969 as a division and continuation-impartof Ser. No. 626,773 and 807,129 and 869,404, and Ser. No. 196,812 filed8 Nov. 1971 as a division of said Ser. No. 839,024.

The present invention relates to composites reinforced by metal fibersparticularly the class of hard metals having high strength and hightemperature use capability (having at least 50 percent room temperaturestrength at 500C and is distinctly advantageous as applied to refractorymetals and of extraordinarily small diameter which may be on the orderof a micron or less, while having continuous length of several timesdiameter and as high as ten inches.

BACKGROUND Metal felts and fine metal wires or fibers or filaments usedin such felts 8re known in the art as indicated in US. Pat. Nos.2,903,787 and 3,178,280. These felts are made from standard cold reducedmetal wires which are limited to minimum diameters on the order of 0.0010.10 inches or less by the inherent vulnerabilities of standard drawingprocesses or from shavings from metal blocks which are characterized bymany surface defects. Much finer wires can be made by extrusion asindicated in U.S. Pat., No. 3,199,331 to Allen. But production by thisprocess is substantially limited as a practical matter to low meltingmetals and alloys (e.g., tin). Other prior art of interest is Buehler,U.S. Pat. No. 3,124,455 and the Speidel, Levi and Wulff work citedbelow.

The present invention involves as a principa object the production'ofmetal fibers of sub-micron size by a new process which is capable ofbeing used with high temperature metals such as tantalum.

It is a further object of the invention to provide an economical methodof making metal fibers on the order of 10 microns or less, andpreferably sub-micron, in diameter with a single series of processingsteps; i.e., free of the expensive supplementary or recycling processinginvolved, for instance, inSpeidel, US. Pat. 3,256,118, Levi, U.S. Pat.No. 3,029,496 and Wulff, January 1966 Journal of Applied Physics, p. 5.

It is a further object of the invention to provide work hardened fibersby a production process free of the need for intermediate anneals asrequired in the above patents of Allen and Levy, and for use incomposites providing a high degree of work hardening in final productform, with or without a final low anneal for stress relief of the matrixonly.

Other objects, features and advantages of the present invention will inpart be obvious and will in part appear hereinafter.

DESCRIPTION The invention is now described with respect to typicalspecific embodiments thereof and with reference to the accompanyingdrawings wherein:

FIG. 1 is a block diagram of the process of the invention.

FIG. 2 is a copy of a photomicrograph of a composite according to theinvention.

FIG. 3 is a copy of a photomicropraph of a metal felt according to theinvention.

FIGS. 4-10 are copies of photomicrographs of a composite according tothe invention.

The fibers of the invention are made and used by the following processdescribed with reference to FIG. 1 which is a block diagram of theprocess. First, powders of the metal to be fibered are obtained. Themetal may be any of the refractory metals (as elements, alloys orcompounds) including tantalum, niobium, molybdenum, tungsten, chromium,beryllium, magnesium oxide, titanium hydride and fabricable aluminidesand silicides. The invention would also be of particular utility anddistinctly advantageous benefit in fibering other hard metal elements,compounds or alloys which have softening temperatures in excess of about1,000C. The starting powder size is variabledependinG upon subsequentprocessing and reactivity of the powders. The invention has beenpracticed successfully for instance with tantalum powders as large asminus 100 mesh and as small as a few microns diameter. The powder isconsolidated into a compact by pressing and sintering or sintering inamold. Then a melt of a second metal is provided in vacuum or inertatmosphere and the powder compact of the first metal is impregnated byeipping in the melt. During both the sintering and impregnating stepsthe compact is degassed and purified to enhance its wettability andductility.

The second metal may be any of aluminum, copper, nickel, Woods metal,tin, indium, mercury, or any other metal which meets the followingcriteria with respect to the first metal under the conditions ofimpregnation:

l. readily wet the skeleton structure of the sintered compact of thefirst metal.

2. not alloy extensively with the first metal.

3. have similar hardness and fabrication characteristics to the extentnecessary for co-working.

4. be easily removable from the compact by chemical or thermal means.

The impregnated compact is then worked down to an this process theadjacent particles of hard metal in the compact begin to form longfibers within the matrix of the second metal.

At this point, the rod or cylinder or plate may be used or fabricatedinto a useful product in any of tht following ways:

A-l. Removing the matrix metal and a. using directly as a filter or withfurther fabrication as a capacitor b. separating out individual fibersc. re-impregnating the fibered article A-2. Using the rod directly as acomposite structural element 8- Rolling the rod to sheet prior to (l) or(2) above C- Drawing the rod to wire prior to (1) or (2) above D-Heating the rod for diffusion reaction between the hard metal fibers andthe matrix prior to (l) or (2) above.

Several permutations of the foregoing can be made. For instance a rodcan be drawn for several passes before rolling. A wire or sheet can beheated for diffusion reaction. Similarly a re-impregnated article can beused as a composite, with or without a diffusion reaction, orre-leached. With diffusion reactions, fibers of alloys or compounds canbe formed even though such alloys are too brittle to be fibereddirectly. Another alternative in the scope of the invention is to form aloose fiber bundle or separate fiber (a or b above) and expose it to anoxidizing or nitriding atmosphere. In this way fibers of aluminum oxideor aluminum nitride can be made for use in reinforced compositestructures. Also fibers of tantalum or niobium nitride can be made foruse as superconductors. In these applications it is of special interestthat the fiber diameters are so small as to favor the formation of theabove compounds in single crystal form which is especially desirable.

The fibers of the invention are characterized in that each fiber isderived from a single powder particle and its length is dependent on thedegree of diameter reduction. For instance, an 8 micron diameter powderparticle fibered to 0.1 microns diameter will have a length of about 1inch, a 30 micron diameter particle fibered to 0.] microns diameter willhave a length of about 70 inches. Further cold working to finer fiberdiameters would increase the length. In most applications of theinvention, useful fibers will have a length of 10 times the diameter ofthe fiber or longer (as high as ltimes for extreme cases).

The felts of the invention are characterized by substantialcross-linking by metallurgical bonds between tangentially contactingfibers corresponding in part to the bonds between powders in theoriginal powder compact skeleton and corresponding in part to new bondsformed during cold working the impregnated compact down to an elongatedarticle, the new bonds being essentially an extension or stretching outof the old bonds.

FIG. 2 shows longitudinal section photomicrograph of a composite in theform of a wire of 0.039 inch diameter at 133 times magnification. Thecomposite has elongated reinforcing tantalum fibers in a matrix ofcopper. The starting material for the fibered metal was coarse meltinggrade powder minus 12 and plus 60 mesh pressed at 18,000 psi andsintered at 2,300C for one hour to produce a compact of 61 percentdensity.

FIG. 3 shows a longitudinal section photomicrograph of a tantalum metalfelt, encapsulated in a molding resin for microscope examination, at 266times magnification. The tantalum was made from nominal 8 microndiameter powders (minus 100 mesh and plus microns) which wasconsolidated to a compact of about 50 percent density and thenimpregnated with copper and then swaged to rod and rolled to sheet afterwhich the copper was leached out in a nitric acid bath. Upon leachingthe metal felt ballooned up to several times its original volume.

Fibers obtained from rod or wire are found to be essentially circular incross-section and fibers obtained from sheet are found to be rectangularin cross-section. The term diameter as used herein refers to diameter ofa circle or width of a rectangle.

The practice of the invention is further illustrated by the followingnon-limiting Examples.

EXAMPLE 1 A mold was filled with tantalum powder of about 8 micronnominal diameter (-100 mesh and plus 5 microns) and the powder wassintered in the mold at 1,500C for 20 minutes to form a green compact.Then sintering was completed by removing the compact from the mold andheating at 2,300C for 1 hour to complete consolidation of the powder.The density of the compact was 8.22 gms/cc or 49.5 percent oftheoretical density. The compact was vacuum impregnated with copper bydipping in a molten copper bath at 1,lC for 5 minutes under a vacuum ofabout 10'' torr. The impregnated compact (0.35 inches diameter by 4inches long) was enclosed in an iron pipe and then swaged to 0.125inches diameter. The jacket was removed and the rod was then furtherswaged to 0.080 inches diameter. After swaging the rod was then leachedin nitric acid to remove the copper. The leached compact left a bundleof interwoven tantalum fibers in the form of a felt.

This metal felt was rinsed and removed from the leach bath. The felt wasanodized and formed into a capacitor anode and tested for capacitorproperties in a wet electrolyte. The formation voltage was 200 volts andthe capacitance was 30.6 microfarads and on a specific weight basis6,l20 microfarad volts per gram. The felt had a dissipation factor of32.19 percent making it an over-all operable capacitor anode.

EXAMPLE 2 Tantalum felts were made as in Example 1 but with thedifference that the compact was rolled to 0.010 inch thich sheet beforeleaching. The felt exhibited a vigorous swelling up with a volumeincrtase and density decrease of 5-10 increase during leaching andfloated on the leaching bath. The capacitor formed from the felt at 150volts had 7,965 microfarad volts per gram specific capacitance.

EXAMPLE 3 Felts were made as in Examples 1 and 2 with the differencethat consolidation of the tantalum powder was accomplished by pressingat 18,000 psi and then sintering at 2,250C for one hour and that somerods were drawn to wire. Densities of 60 percent of theoretical wereobtained in the original compact. Upon leaching the final compositearticle of this type, the felt did not swell up. However, high values ofcapacitance were still obtained indicating substantial formation of newsurface as in Examples 1 and 2 (surface enhancement of about 2.5 times).

EXAMPLE 4 Several fibers from the felts of Examples 1 and 2 wereencapsulated in epoxy resin and measured to yield an individual fiberdiameter indication of 0.0002 cm. diameter. The Example 2 fibers were 5to 10 times as long as the diameter of the fiber; the Example 1 fiberswere continuous over much longer length. 4

EXAMPLE 5 Several compacts made essentially as in Examples 1 and 2 wererolled or drawn to the final sizes indicated below for testing of theircomposite material properties. These tantalum reinforced coppercomposites were in the form of 0.020 inch diameter wire and as 0.010inch thick sheet, both as worked andafter being heated (350C for 1 hourto anneal the copper). The results for these specimens and forcomparison, the

properties of tantalum and copper, per se, are given in Table 1:

TABLE 1 Example 5 Sample a. 0.01-0.020 inch diameter wire as workedUltimate Tensile Strength 160,000-195,000 psi b. wire with stress relief150,000-172,000 c. sheet, as worked 99900-127900 d. sheet, stressrelieved 93,000 e. Pure tantalum, as worked (0.005 and 0.015 inch thicksheet) 104,000-1 16,000 f. Pure copper, as worked (0.005

and 0.015 inch thick sheet) 59,00060,500

EXAMPLE 6 A molybdenum copper composite was made and tested in the samemanner as the tantalum copper composites of Example 5 and formed into0.06 and 0.08 in wire which displayed ultimate tensile strengths of81,700 and 108,000 psi, respectively.

EXAMPLE 7 Tantalum felts made as in Examples 4 and 2 were tested fortensile strength after leaching out the copper. The results are in Table2.

TABLE 2 Example Sample Ultimate Tensile Strength a. 0.01 in sheet 114,700 psi b. 0.04 in wire 90,000 psi EXAMPLE 8 EXAMPLE 9 Beforeleaching, the iron copper composite wire of Example 8 was tested fortensile strength and this was found to be 160,000 psi.

EXAMPLE l Leaching experiments were conducted and a solution of partsammonium hydroxide in one part hydrogen peroxide was found to besuperior to nitric acid for selectively leaching copper from the iron tofree the iron fibers from the composites.

EXAMPLE 1 l A sample of l00/+325 mesh tantalum powder was placed insideof a sealed rubber tube and was isostatically pressed at 3,000 psi. Thepressed powder bar (1 inch diameter rod) was then vacuum sintered at2,200C for 2 hours to give a theoretical density of 45 percent. Thesintered porous powder bar was infiltrated with superheated copper togive a composite of l inch diameter. The Ta-Cu composite was swaged to0.2 inch in diameter and was sheared into V4 inch slugs. Copper was thenleached out from the cut ends of the :4 inch slugs leaving an array oftantalum fiber ends. The leached cut ends were then examinedmicroscopically.

FIG. 4 showed a low magnification (29X) photograph of the sheared end ofa fiber tantalum slug FIGS. 5, 6 and 7 were magnified (700X)photomicrographs of regions A, B and C marked in FIG. 4 respectively.These were taken end-on from a viewing angle of 15 with respect to therod (slug) axis. FIGS. 8 and 9 were two different magnifications (670Kand 2,650X, respectively; FIG. 9 being a blow-up of the circled area ofFIG. 8 note the corresponding die marks) of the side surface of theslug. FIG. 10 was an inside longitudinal section view at 530x of theslug located half a radius distance from center to periphery of theslug.

The slugs were cut from top to bottom of the FIG. 4 view and ductilefailure is apparent at the bottom edge of the slug (region C). Fibers ator near the slug peripheries are of tobacco-leaf form reflecting thecharacteristic twisting of swaging operations. But fibers below thesurface are filamentary in appearance and are believed to be round incross-section at the center of the slugs (note FIGS. 8 and 9 for surfaceand subsurface fibers). All figures (except low magnification FIG.4)-show the essentially straight axial orientation of the fibers.Interconnections can also be seen in the side views.

The best mode of using the invention is believed to be selection of atantalum copper pair to produce a tantalum felt suitable for use as acapacitor anode. In addition to the above indicated advantages of easeof processing, surface enhancement and work hardening it is a furtheruseful advantage of the invention that it may be practiced if desired,with relatively coarse melting grade tantalum powder in the originalcompact rather than the conventional fine grain capacitor grade powderand the desired surface area increase can be obtained in the fiberingp7ocess rather than in the original processing of the pooder. A furtheruseful aspect of the invention is the above described feature ofswelling when the original compact is made in low density (40 percenttheoretical) and/or when a high degree of working is put into thecomposite. The swelling of the metal felt, when utilized makes it easierto refill the felt with an anodizing medium and electrolyte.

The extension to other species of the above advantages and variations inprocessing and still other advantages and variations will be obvious tothose skilled in the art from the description herein. For instance, aniobium tin pair could be utilized to 4btain interconnected niobiumfibers in a tin matrix with a better degree of interconnection betweenfibers than is obtainable in the process of the above described Speidelpatent. Then the composite could be heated for diffusion reaction toform a niobium stannide superconductor subsequent to which residual tinwould be leached out and replaced with copper by re-irnpregnation toprovide a higher conductivity matrix for electrical stability of thesuperconductor.

A high degree of control of the final product is obtainable. Forinstance, use of coarse melt grade powders or low density consolidationof the original compact (40 60 percent) tend to limit the number ofcross-link bonds formed between fibers thereby enhancing the swelling upof fibers upon leaching the matrix metal and enhancing the ease ofseparation of fibers.

For superconductor applications it is particularly desirable to use afine grain powder and form the original compact to a higher density forforming maximum cross-links between fibers. Still other applicationswithin the scope of the present invention will be apparent to thoseskilled in the art when aided by the foregoing description. Thedescription is therefore intended to be read as illustrative and not ina limiting sense.

What is claimed is:

1. An elongated composite material comprising a fibrous reinforcingcomponent of a first metal in a matrix of a second metal, said firstmetal being a refractory metal in elongated fiber form with an averagelength of at least 0.025 centimeters and said reinforcing componentcomprising an elongated felt bundle of said fibers axially oriented inthe direction of the long dimension of the fibers, the fibers beinginterconnected to each other be metallurgically bonded cross-linksspaced along the fiber lengths, said second metal comprising a metalwhich is compatible with said first metal.

2. The composite material of claim 1 wherein said fibers were cold-workstrengthened to the point that the elongated composite has a tensilestrength at least equal to an equivalent volume and cross-section of anelongated member of the first metal alone.

3. The composite material of claim I wherein the fibers have an averagediameter of less than 0.00025 centimeters.

4. The composite material of claim 1 as produced by forming a powdermetallurgy compact of the first metal with sintered bonds betweenpowders and interconnected open space within the compact, inpregnating acompatible matrix metal in molten form into the open space to form amatrix for the first metal cooling the molten phase to solid,mechanically working the impregnated compact to an elongated form andfibering the powders while so doing.

5. The composite material of claim 4 wherein said second metal is theoriginally impregnated matrix metal.

6. The composite material of claim 4 as produced by removing theoriginally impregnated matrix metal after elongation and substitutingsaid second metal therefor through impregnation of the fiber bundle withsaid second metal.

7. The product of claim 1 wherein the fibers comprise niobium stannideand the matrix comprises copper.

8. The product of claim 1 wherein the fibers comprise tantalum and thematrix comprises copper.

9. The product of claim 1 wherein the fibers comprise molybdenum and thematrix comprises copper.

10. The product of claim 1 wherein the fibers comprise tungsten and thematrix comprises copper.

11. The product of claim 1 w erem the felt has a swollen arrangement toexhibit a density of less than half of the theoretical density of themetal.

12. The product of claim 1 wherein the felt has a compact arrangementwith a density over half of the theoretical density of the metal.

Disclaimer 3,729,794.Riehm'd W. Douglass, Needham, Mass. FIBERED METALPOW- DERS. Patent dated May 1, 1973. Disclaimer filed May 2, 1973, bythe assignee, N orton Oompany.

Hereby disclaims the portion of the term of the patent subsequent toAug. 1, 1989.

[Ofiee'al Gazette December 4, 1.973.]

Disclaimer 3,729,794.Ri0hm"d W. Douglass, Needham, Mass. FIBERED METALPOW- DERS. Patent dated May 1, 1973. Disclaimer filed May 2, 1973, bythe aSSignee,N01'2/0n Company.

Hereby disclaims the portion of the term of the patent subsequent toAug.

[Oficial Gazette. December 4, 1973.]

2. The composite material of claim 1 wherein said fibers were cold-workstrengthened to the point that the elongated composite has a tensilestrength at least equal to an equivalent volume and cross-section of anelongated member of the first metal alone.
 3. The composite material ofclaim 1 wherein the fibers have an average diameter of less than 0.00025centimeters.
 4. The composite material of claim 1 as produced by forminga powder metallurgy compact of the first metal with sintered bondsbetween powders and interconnected open space within the compact,inpregnating a compatible matrix metal in molten form into the openspace to form a matrix for the first metal cooling the molten phase tosolid, mechanically working the impregnated compact to an elongated formand fibering the powders while so doing.
 5. The composite material ofclaim 4 wherein said second metal is the originally impregnated matrixmetal.
 6. The composite material of claim 4 as produced by removing theoriginally impregnated matrix metal after elongation and substitutingsaid second metal therefor through impregnation of the fiber bundle withsaid second metal.
 7. The product of claim 1 wherein the fibers compriseniobium stannide and the matrix comprises copper.
 8. The product ofclaim 1 wherein the fibers comprise tantalum and the matrix comprisescopper.
 9. The product of claim 1 wherein the fibers comprise molybdenumand the matrix comprises copper.
 10. The product of claim 1 wherein thefibers comprise tungsten and the matrix comprises copper.
 11. Theproduct of claim 1 wherein the felt has a swollen arrangement to exhibita density of less than half of the theoretical density of the metal. 12.The product of claim 1 wherein the felt has a compact arrangement with adensity over half of the theoretical density of the metal.